A single narrow pulse of light may consist of many wavelengths in a given passband entering a fiber optic transmission system. During the travels through the fiber optic transmission system that single narrow pulse of light becomes dispersed, separated in time, due to effects of chromatic dispersion. Therefore, a chromatic dispersion module's overall goal is usually to delay wavelengths in a given passband enough to combine all the wavelengths in the passband into a single narrow pulse.
Chromatic dispersion is pulse spreading arising from differences in the speed that light of different wavelengths travel through a material, such as fiber optic cable. Chromatic dispersion is the variation in the propagation speed of light as a function of wavelength. Chromatic dispersion causes a distortion of the optical pulses that propagate through a fiber optic transmission line. As noted, to compensate for the chromatic dispersion in the fiber spans, chromatic dispersion compensating modules (DCMs) are placed periodically in the transmission line. Chromatic dispersion compensating modules add dispersion to the signal, which is ideally equal and opposite in sign, to counteract the dispersion accumulated in the fiber span. The pulses are then reformed to counteract and eliminate the chromatic dispersion-induced distortion within a passband of wavelengths.
In prior technologies, all-pass filters have been tried in dispersion compensation devices. FIG. 1 illustrates a block diagram of a prior art basic etalon-type all-pass filter. The backside mirror has 100% reflector while the front side mirror can have a reflectivity less than 100%. The term all-pass means that no fundamental sources of loss in the device exist, and thus, the theoretical amplitude response equals unity at all wavelengths. The etalon-type all-pass filter therefore only affects the phase of the light. Because dispersion is a change in the phase of the light, this type of filter is well suited to chromatic dispersion compensation.
In FIG. 1, the light travels into the basic etalon-type all-pass filter. A combined input/output fiber sends an optical signal into a collimating lens. The light is collimated and sent at normal incidence into the basic etalon-type all-pass filter. The basic etalon-type all-pass filter produces a variation in the time delay due to the resonate circulation of some wavelengths within the cavity. For wavelengths that are at resonance, the light effectively stays inside the cavity longer than for wavelengths that are off resonance. This causes a wavelength-dependent delay that produces dispersion. Light traveling out of the fiber eventually returns to the collimating lens. However, these basic etalon-type all-pass filter may have unacceptable high amount of insertion loss. Insertion loss is measured by the strength of the optical signal coming out of the all-pass filter as compared to the strength of the optical signal entering the all-pass filter.
Another previous technique of trying to compensate for dispersion compensation was to add a dispersion compensation fiber to a group of optical amplifiers. However, the dispersion compensation fiber was not really adjustable in length or amount of dispersion, and also incurred heavy insertion losses. The heavy insertion losses were some of the non-beneficial effects of adding the dispersion compensation fibers to the system.
Another previous parameter of trying to compensate for dispersion compensation to make the device temperature independent or maintain a temperature so that the variable of temperature does not effect the operation of the device. However, in some cases it might be advantageous to take advantage of temperatures affect on the refractive index of optical components.
Also, most of the above devices are able to tune the dispersion compensation while maintaining a constant dispersion slope. However, in order to compensate for the dispersion of arbitrary fibers, or arbitrary combinations of fibers, a dispersion compensation device that cannot adjust the dispersion and dispersion slope independently may be inadequate.